The invention relates to an apparatus for measuring the particulate matter in the flue gas or exhaust gas from a combustion process.
In a measuring instrument of this kind, the light emitted by an optical transmitter passes through the exhaust pipe and is converted by an optical receiver into an electrical signal. The particles located in the exhaust gas lead to a decrease in the amount of light, which is expressed as extinction or turbidity and is a standard for the soot emission of a combustion device.
In a known apparatus of this kind (German Utility Model 81 28 634), bodies transparent to light are made in the form of windows, each closing off a passage, inserted into the light opening or slit, spaced apart from the tubular wall of the exhaust pipe. The result is a hydraulically idle volume in front of each window, which causes considerable deposits of particles on the windows. To eliminate the measurement errors originating in the dirt on the windows, one window has a retroreflector associated with it, which is protected in a chute against being soiled and is located outside the beam of light. At certain time intervals, the reflector is briefly moved into the beam path. It then reflects the beam of light from the optical transmitter that passes through the window back to the optical receiver. There a signal is obtained that is equivalent to the reduction in transmission caused by soiling of the window and provides a correction value for further measurement once the retroreflector is removed. This kind of recalibration of the measuring instrument is complicated and is suitable only for stationary combustion systems, but not for mobile ones such as internal combustion engines in motor vehicles.
In internal combustion engines equipped with fuel injection pumps, such measuring equipment is used so that from the soot concentration in the exhaust gas, a control variable for fuel injection is derived, with which the full-load injection quantity can be metered in such a way that the maximum soot emission prescribed by law is not exceeded (U.S. Pat. No. 3,744,461).
In a known measuring instrument for internal combustion engines (British Patent 1,334,472), an electrical detector is installed on the circumference of the exhaust pipe carrying the flow of exhaust gas, and the detector receives the signal from an electrical transmitter that is likewise disposed on the circumference of the exhaust pipe, directly opposite the detector. The intensity of the signal received by the detector increases or decreases--depending on the type of detector--whenever the soot concentration in the exhaust gas flow increases. The electrical output signal of the detector is amplified and forms a direct control signal for adjustment of a valve. For maximum prevention of adulteration of the measurement from soot concentration on the detector and transmitter, a curtain of flushing air is installed between the exhaust gas, on the one hand, and the active elements of the detector and transmitter, on the other. Nevertheless, soot deposits on the active elements cannot be prevented, so that over the long term a drift in the measurement values occurs.